1. Field of the Invention
This invention relates to a GaN crystal film formed by epitaxial growth on a sapphire substrate, and a semiconductor device fabricated using the GaN film. This invention also relates to a group III element nitride semiconductor wafer formed by epitaxial growth on a heterogeneous-material substrate (referred to as a "hetero-substrate"), a semiconductor device fabricated using the group III element nitride semiconductor wafer and a manufacturing process therefor.
2. Description of the Related Art
GaN has a large forbidden bandgap of 3.4 eV and is a direct-transition type of compound semiconductor, and thus has been attractive as a blue-light emitting device material.
A material for a substrate in fabricating a light-emitting device using the GaN material is preferably a bulk crystal of the same material as an epitaxial layer to be grown, i.e., a bulk crystal of GaN. It is, however, difficult to form a bulk crystal of GaN due to a higher dissociation pressure of nitrogen; that is, it is very difficult to fabricate a GaN-bulk crystal substrate. Therefore, a sapphire (Al.sub.2 O.sub.3) substrate having a relatively close lattice constant to GaN has been used, on which GaN is epitaxially grown. Thus, a material whose physical properties such as a lattice constant and coefficient of thermal expansion as well as chemical properties are quite different from those of an epitaxial layer has been used for a substrate.
It has been described in Jpn. J. Appl. Phys. Vol.32 (1993), pp.1528-1533 that epitaxial growth on such a hetero-substrate may cause strain or defects and, if growing a thick film, cracks. In such a case, there has been often caused a device with an extremely poor performance or a shattered growth layer.
J. Mater. Res. Vol.11 (1996), pp.580-592 has described a correlation between a dislocation structure in a GaN film grown on a sapphire substrate and its crystal quality.
It has described that (a) a GaN film on a sapphire substrate is formed via mutual coalescence of island crystal grains c-axis oriented in parallel with the normal line of the substrate surface, (b) during it growing, individual crystals rotate by a small angle centering the c axis, to form dislocations in an interface between crystal grains, (c) these dislocations cause threading dislocations having a displacement vector parallel to the c plane of the GaN crystal.
Presence of such threading dislocations in the GaN crystal film means that the GaN crystal film has a domain form divided by original crystal grains, and a crystal orientation component parallel to the c plane of the GaN crystal film have different structure for each domain. It may reflect that the GaN crystal film forms a mosaic structure, which means that the density of the dislocations having a displacement vector parallel to the c plane must be minimized as much as possible for improving the crystal quality.
To solve the problems, JP-A 8-64791 has disclosed a process for epitaxial growth of a lattice mismatch system wherein dislocations generated due to lattice mismatch between a substrate and an epitaxial growth layer are concentrated to a particular area. It has described that the process may minimize a dislocation density in a desired region, which makes it possible to fabricate a semiconductor light-emitting device for an application such as a semiconductor laser requiring a high quality crystallinity. Specifically, an amorphous GaN film is formed on a sapphire substrate in the first crystal growth, the film is etched into a stripe, and a GaN film is epitaxially grown on the amorphous GaN film and the substrate in the second crystal growth. In another example, an SiO.sub.2 film in place of the amorphous GaN film is formed in a stripe and an epitaxial layer is grown only on the substrate.
Such a process, however, forms an amorphous region on a surface, so that a homogenous growth layer cannot be formed over all the surface or there is generated a region where no growth occurs on an SiO.sub.2 area, so that a flat growth layer cannot be formed over all the surface. Thus, there are limitations for an area on which a device is to be formed.
In addition, a substrate material such as sapphire, silicon carbide and MgAl.sub.2 O.sub.4 has different lattice constant, crystal structure and coefficient of thermal expansion from those in a GaN epitaxial growth film, which gives a serious problem of curving in an epitaxially formed wafer. For example, when a sapphire substrate is used, significant curving may be generated as schematically shown in FIG. 14. A substrate with a diameter of 1 inch may be curved in a level that its center is convex by several millimeters with respect to its periphery; the radius of curvature may be below 70 cm.
The problem of curving cannot be improved very much even when a different type of substrate material is used or a mixed crystal such as AlGaN and InGaN or a group III element nitride semiconductor such as AlN and InN was epitaxially grown in place of GaN. Therefore, epitaxial growing of GaN on a sapphire substrate will be described.
If there is a significant curvature of a wafer, it may cause, for example, difficulty in applying a lithography technique to a later device formation. When there is a considerable curvature, a wafer should be divided at least prior to applying lithography in a manufacturing process. For example, the wafer should be divided into areas of about 5 mm.sup.2 before applying lithography to form a window for a stripe laser for electric current injection.
A blue-light-emitting optical device is one of applications of a group III element nitride semiconductor having a wurtzite crystal structure. In particular, there is much prospect that the semiconductor may realize a digital video disk (DVD) using a blue laser as a light source which enables data to be written or read out in a high density. A Fabry-Perot resonator of such a semiconductor laser is generally formed by cleavage. It may be, for example, assumed that a GaN epitaxial layer is formed on a sapphire substrate, on which there is, then, epitaxially formed a double-hetero (DH) structure for a laser with a group III element nitride semiconductor where a group III element is nitrogen, to form a stripe structure, solving the above problem in lithography. Other components such as an electrode may be formed in subsequent steps, and finally a Fabry-Perot resonator should be formed generally by cleavage.
However, when there is a significant mismatch between the cleavage planes of the GaN epitaxial layer and the sapphire substrate, it may be quite difficult to obtain a clear cleavage in the presence of the sapphire substrate. Thus, before cleavage, it may be necessary to remove the sapphire substrate by, for example, grinding. In other words, due to the problem for lithography, there is added a complicated process that the sapphire on the rear face of the wafer divided into small areas is ground.
Furthermore, even if it is possible to conduct cleavage leaving the sapphire substrate, there may remain the sapphire substrate 101, an insulating material, on the rear face, and therefore, an electrode cannot be formed on the rear face. Thus, there may be added a process that an electrode 102 is formed on a partial area excavated from the surface of the DH structure (an electrode-forming layer 106). That is, the electrode 102 instead of a rear electrode should be formed as shown in FIG. 15(a) (a conceptual cross section of a laser structure viewing from the cross-section of the Fabry-Perot resonator, where 104 is a silicon oxide film and 105 is a laser emission region). On the other hand, if the sapphire substrate 101 is removed, the electrode 102 can be disposed on the rear face, facing the surface electrode 103 as shown in FIG. 15(b).
Furthermore, the problem of curving due to a thick dissimilar material substrate such as sapphire on a wafer, may occur when epitaxially growing, for example, a DH structure on the wafer. Specifically, a significantly curved wafer may not be easily placed in a holder and may cause a temperature difference within the wafer surface during epitaxial growth because the holder cannot be in contact with the whole surface of the wafer.